The removal of N and P in aerobic and anoxic-aerobic digestion of waste activated sludge from biological nutrient removal systems

Abstract

Biological nutrient removal (BNR) activated sludge (AS) systems produce a waste activated sludge (WAS) that is rich in nitrogen (N) and phosphorus (P). When this sludge is thickened to 3–6% total suspended solids (TSS) and digested (aerobic or anaerobic), a high proportion of N and P are released to the bulk liquid resulting in high concentrations of ammonia/nitrate and orthophosphate up to several hundred mg/ℓ (without denitrification or P precipitation). This research investigates P removal by P precipitation in anoxic-aerobic digestion of P-rich BNR system WAS. The experimental setup for this work was a lab-scale membrane UCT BNR system fed real settled sewage with added acetate, orthophosphate, and cations Mg and K to increase biological excess P removal. This WAS was fed to batch aerobic digesters at various TSS concentrations, and to two 20-day retention time continuous anoxic-aerobic digesters (AnAerDig) with aeration cycles of 3-h air on and 3-h air off, one fed concentrated WAS (20 g TSS/ℓ ) and the other fed diluted WAS (3 g TSS/ℓ). Nitrogen removal has been discussed in the previous paper. This paper focuses on the P removal by P precipitation observed in the batch tests and continuous systems. The rate of polyphosphate release (b_GP) during batch aerobic digestion at low TSS without P precipitation was found to be 2.5 times faster than the endogenous respiration rate (b_G) of phosphorus accumulating organics (PAO), i.e. b_GP = 0.1/d. This rate was then applied to the high-TSS aerobic batch tests and continuous anoxic-aerobic digesters to estimate the P precipitation at various TSS concentrations, with and without additional Mg or Ca dosing. Newberyite (MgHPO₄·3H₂O) and amorphous tricalcium phosphate (ACP or TCP, Ca₃(PO₄)₂·xH₂O) are found to be the most common phosphate precipitates.

Keywords: biological excess phosphorus removal, waste activated sludge, anoxic-aerobic digestion, phosphaterelease, mineral precipitation